An individual's ability to modulate arousal, attend to and affectively respond to stimulation during social interaction is critical to social development and mental health. Disturbances in these functions have been noted during social interactions as early as infancy. Behavioral and psychophysiological data suggest that some infants (e.g. preterm, postterm, Down's syndrome, autistic and hyperactive infants) may have abnormally high or low sensory and defensive thresholds to stimulation and may have difficulty modulating arousal, attending and positively responding except to a very narrow range of stimulation during their early interactions. The purpose of the proposed research is to assess this hypothesis in a number of spontaneous and manipulated interactions by varying the type and amounts of social stimulation, monitoring a multivariate complex of behavioral and psychophysiological measures and using multivariate analyses to better understand the functional significance and relationships of social behaviors such as smiling, laughter and gaze aversion. The research will assess both normal and high-risk infants in a longitudinal series of studies, and in addition to describing the relationships predicted by the model, will explore intervention strategies to facilitate arousal, attentional and affective processes.